1. Technical Field
The present disclosure relates to a battery system for an electric vehicle, and in particular, relates to a battery system including a nonaqueous secondary battery for a vehicle drive power supply.
2. Description of Related Art
A nonaqueous secondary battery having a nonaqueous electrolyte, which is represented by a lithium-ion secondary battery, is used for a vehicle drive power supply (on-vehicle power supply) of an electric vehicle, such as a hybrid car, an electric car, or a fuel battery car. As one factor for causing an increase in resistance of the nonaqueous secondary battery, a phenomenon in which high rate charging or discharging is continuously performed to cause an increase in resistance due to unevenness (deviation) of a salt concentration (ion concentration) in the electrolyte, so-called high rate deterioration, is known.
International Publication No. WO 2013/046263 describes that an integrated evaluation value (ΣDex(N)) is determined as a deterioration index value by integrating an excess of a threshold for an evaluation value (D(N)) of deterioration due to deviation of an ion concentration in an electrolyte calculated based on a history of a current value when a battery is charged/discharged and correcting relaxation of deviation of the ion concentration with time. Then, control for reducing a discharge power upper limit of the secondary battery when the deterioration index value exceeds a threshold (positive).
Japanese Patent Application Publication No. 2014-3826 (JP 2014-3826 A) describes that a common integrated evaluation value based on the same evaluation value as in WO 2013/046263 is used as a deterioration index value, if the integrated evaluation value becomes greater than a positive threshold, it is determined to be over-discharged and the discharge power is restricted, and if the integrated evaluation value is smaller than a negative threshold, it is determined to be over-charged and the charge power is restricted.
WO 2013/046263 and JP 2014-3826 A describes that, in calculating the evaluation value (D(N)) of deterioration due to deviation of the ion concentration, calculation reflecting a decrease in deviation of the ion concentration with ion diffusion at each cycle time (Δt) is performed.
In addition, WO 2013/046263 describes that, if charging/discharging of the secondary battery is deactivated, an increase (that is, high rate deterioration) in resistance increase rate due to charging/discharging resumed after deactivation is suppressed. For this reason, WO 2013/046263 describes that calculation for multiplying the integrated evaluation value used as the deterioration index value by a correction coefficient a (0<a<1) is executed at each cycle time, thereby correcting the integrated evaluation value (deterioration index value) in consideration of relaxation of high rate deterioration.
However, in the techniques described in WO 2013/046263 and JP 2014-3826 A, it is determined whether or not there is a need for restricting the discharge power or the charge power by comparing the integrated evaluation value (deterioration index value) and the threshold at each cycle time (Δt) at which the evaluation value (D(N)) of deviation of the ion concentration and the integrated evaluation value (ΣDex(N)) are calculated. Accordingly, WO 2013/046263 and JP 2014-3826 A is primarily directed to charging/discharging management in a period during which the secondary battery is used (charged/discharged), such as during vehicle traveling.
In recent years, an electric vehicle which is able to charge an on-vehicle power storage device with a power supply (hereinafter, simply referred to as “external power supply”) outside a vehicle comes into practical use. In a so-called plug-in hybrid car or an electric car, charging (hereinafter, simply referred to as “external charging”) of an on-vehicle secondary battery with an external power supply is performed using a system power supply or a dedicated charging stand during the stop of operation.
In such an externally chargeable electric vehicle, a use mode of the secondary battery from the stop of operation stop to the next start of operation changes with the presence or absence of external charging. Specifically, the length of the charging/discharging deactivation period of the secondary battery changes with the presence or absence of external charging. Furthermore, even in a case where a charging schedule is established such that external charging is completed in conformity with the next start time of operation by so-called timer charging, it is predicted that the length of the charging/discharging deactivation period of the secondary battery from the stop of operation to the start of external charging changes according to situations.